Growth of crystalline WO3-ZnSe nanocomposites: an approach to optical, electrochemical, and catalytic properties

In this study, novel growth of WO3-ZnSe nanocomposites was carried out by a simple, low-cost hydrothermal process under subcritical conditions and is reported for the first time in just 5 h. The products were characterized in detail by multiform techniques: X-ray diffraction, scanning electron microscopy (SEM), optical studies, and Fourier transform analysis. The influence of ZnSe on the structural, morphological, compositional, optical, and catalytic properties of WO3 is demonstrated. The WO3 metal oxide material is grown in a hexagonal crystal structure with wide-band-gap and has been modified by ZnSe to form a composite nanostructures in the nanoscale range. The electrochemical properties of the prepared materials were studied by cyclic voltammetry, which revealed that the synthesized material exhibited remarkable electrochemical supercapacitive activity. Moreover, the composite nanostructures showed excellent photocatalytic activity for degradation of phenol and almost 93% of phenol was degraded with good recyclability and stability. According to The International Commission on Illumination (CIE), the synthesized nanomaterial shows blue emission and is suitable for blue LEDs.

gas detecting gadgets 9 . Moreover, a new report has shown that tungsten oxide has noteworthy biophotocatalytic properties alongside its nonperilous nature that has shown its potential applications in nanobioinnovation [14][15][16][17][18] . Zinc Selenide (ZnSe) was developed as the most encouraging photocatalytic material with a band gap of 2.7 eV at room temperature and enormous excitation restricting energy of 21 meV. Recently, the nanostructures of ZnSe have received much consideration because of their outstanding redox properties. Ongoing investigations showed that these ZnSe nanostructures display great photocatalytic activities and retain the edge of apparent light.
To improve the activity of these individual nanostructures, the development of ZnSe-WO 3 nanocomposite was performed, which not only improves the absorption of visible light but also efficiently isolates photogenerated charges. The synthesis of heterojunctions with suitable arrangement of conduction bands and valence bands prompt diminished recombination of charge carriers [19][20][21][22] . Thus, overlapping band edges at the interface promotes charge transfer and higher light absorption. Numerous biomolecule-supported materials, such as chitosan-GO 23 , and chitosan-TiO 2 24 and heterojunctions, such as ZnO-TiO 2 , TiO 2 -Cu2S 25 , and ZnO-ZnSe 26 , are being utilized to remove environmental pollutants through adsorption and photocatalysis.
In the present study, we report the cost-effective synthesis of WO 3 -ZnSe nanocomposites and have applied them for multiple applications, such as electrochemical, photoluminescent, and photocatalytic applications. The fabrication of composite material has an advantage over individual counterparts in light absorption, redox activity and interfacial charge transfer. The photocatalytic activity was checked with degradation of phenol and in electrochemical activity, supercapacitive behavior was checked. The WO 3 -ZnSe composites showed excellent photocatalytic and good capacitive behavior with 93% of phenol degradation in 105 min.
Fabrication of the WO3-ZnSe nanostructures. All the substances were analytic-grade reagents without further distillation.WO 3 nanoparticles were synthesized under hydrothermal conditions. The experimental details were as follows: 2 g sodium tungstate dihydrate (Na 2 WO 3 ·2H 2 O) was dissolved in 40 mL of distilled water. HCl solution was added dropwise until the color changed from transparent to light yellow with constant stirring for 30 min. This colored liquid was then transferred into a Teflon-coated autoclave and put in a hot air oven for 4 h at 180 °C. Then cool the former at room temperature. The sample was then filtered and washed several times with ethanol and distilled water. The collected sample was dried in an oven at 50 °C.
ZnSe nanostructures were synthesized via a hydrothermal approach as reported earlier 27 . The WO3-ZnSe nanocomposite was prepared by the hydrothermal method. The preparation was performed by mixing WO 3 nanoparticles into the ZnSe solution. The whole sample was transferred into a Teflon-coated autoclave in a hot air oven at 180 °C for just 5 h. Cool the autoclave naturally and filter the particles. The collected sample was washed several times with condensed water and ethanol. Dry the particles for the characterization part.
Characterization techniques. The Rigaku Miniflex 600 diffractometer was used to perform the diffraction patterns of the synthesized material. In order to see the particle size, morphology and elemental composition analysis scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) was employed, by a field emission-scanning electron microscope (FESEM) (Philips Model-Quanta 200 FEG). For optical studies an UV-Visible spectrophotometer (UV2450 Shimadzu) was used. For photoluminescence, a spectrofluorophotometer (RF 6000, Shimadzu) was employed.

Results and discussion
The crystal structure of the composite material was investigated by X-ray diffraction. Figure 1 shows the XRD patterns of the ZnSe, WO 3 , and WO 3 -ZnSe nanocomposites. The peaks for ZnSe are indexed and perfectly match with the cubic phase. The lattice parameters were calculated to be a = 5.5 Ǻ and matched well with the reported values for cubic ZnSe 27   . The maximum of the major peaks are sharp, which represents the high crystallinity of WO 3 nanoparticles. No, hydrated WO 3 peaks were found in the sample, indicating the high purity of WO 3 . The average grain size of the synthesized nanostructures was found to be between 3 and 30 nm, shown in Table 1 and was calculated by using the Scherer formula.
Here, D is the average size of the nanoparticles, n is the dimensionless shape factor (0.9), λ is the wavelength of incident X-ray (λ = 1.54 Å), β is the full width at half maximum (FWHM) of the diffraction peak and θ is the angle of diffraction.
The XRD pattern of the WO 3 -ZnSe nanocomposite, confirms the presence of both WO 3 and ZnSe. No, important transformations are seen in the XRD patterns of the pure WO 3 and WO 3 -ZnSe nanocomposites. This means that the phase purity of WO 3 is maintained after composite formation with ZnSe, which is preferred for photocatalytic activity. Figure  SEM and TEM analysis. The morphology and particle size of the sample were studied by scanning electron microscopy. Figure 3 represents the SEM micrographs of ZnSe (a), WO 3 (b) and WO 3 -Znse (c) nanoparticles. WO 3 nanoparticles have an irregular structure or solid rock-like structure while ZnSe particles are spherical. It is clear from the SEM images that the spherical nanostructures are dispersed on the surface of WO 3 nanoparticles, hence increasing the surface area of the synthesized material, which enhances the catalytic properties of the material. The outer state and morphology of the sample can play a major role in improving photocatalytic activity. When the electrons and holes are surrounded in surface states, the nonlinear overlaps of charge carriers are reduced, and their recombination is further stunted due to the localized nature of the surface area. In the present study, the formation of associative rock-like WO 3 and spherical ZnSe particles will lead to the formation of efficient photocatalyst for the interfacial charge transfer and separation of photoexcited electron-hole pairs. Thus, the synthesized material can be favorable for photocatalytic activity. The further insight into the surface morphology and structure was ascertained by Transmission electron microscopy (TEM). It is clear from Fig. 3d that spherical particles of ZnSe are irregularly dispersed on the surface of WO 3 nanoparticles. The shapes of particles are clearly seen to be spherical anchored on rock like nanostructures having dimensions in the nanometer scale. The irregular dispersion of crystalline ZnSe particles on the surface of WO 3 nanostructures may increase the surface active sites and enhance catalytic activity.

FTIR analysis.
To determine the elemental composition of the fabricated samples, energy dispersive spectrometry (EDS) was employed. The EDS arrangements and elemental composition of ZnSe, WO 3 and WO 3 -ZnSe nanoparticles are shown in Fig. 4a-    Electrochemical analysis. Electrochemical analysis, such as CV and specific capacitance measurement, were used to study the charge-storage performances of ZnSe, WO 3 and WO 3 -ZnSe nanocomposites in threeelectrode setups displayed in Fig. 6. The fabricated samples were homogeneously dropped cast on the working electrode (made up of glassy carbon) at room temperature. Ag/AgCl and platinum wires were used as reference and counter electrodes, respectively. All three electrodes were dipped in 0.5 M H 2 SO 4 solution, which is used as an electrolyte in an electrochemical cell. Figure 6 display typical CV plots from 10 to 50 mVs −1 . The pair of redox reactions that are unseen in electrical double-layer capacitors is obvious in the CV curve and corroborate its battery type behavior, which is the significant requirement of the pseudocapacitive mechanism 11,33,34 . The studies also showed that the redox reaction is a diffusion-controlled progression, which may be the main cause of the pseudocapacitive behavior with increasing scan rates. Cyclic voltammetric curves are affected by diffusioncontrolled and kinetic-controlled processes. If a redox process is only affected by diffusion, the peak potential should generally be independent of the scan rate. If the electrode kinetics is dominant, the peak potential is affected by voltammetric changes due to increasing scan rates. The shape of CV curves change little and the area enclosed increases with the increasing of scan rate, this demonstrates that the WO 3 -ZnSe nanocomposites have good electrical conductivity and reversibility. WO 3 nanoparticles showed a less capacitance but after making composites with ZnSe, the relative specific capacitance increased by a large amount which are clearly shown in the Fig. 6.
Electrochemical impendence spectroscopy analysis. Electrochemical impedance spectroscopy (EIS) was done to study the rate of interfacial charge transfer and dynamic kinetics of electrode reaction. The Nyquist plots for ZnSe, WO 3 , and WO 3 -ZnSe nanocomposites are shown in Fig. 7. Impedance is the power that contradicts electrical flow, and it is estimated in units of resistance (Ω). The genuine part (Z) versus the fantasy part (Z) of the impedance, which selects the interphase resistance between the functioning cathode and  www.nature.com/scientificreports/ the electrolyte, was employed to plot EIS spectra. The R ct of composite WO 3 -ZnSe was found to be lower than pure oxide and selenide counterparts. The lower resistance of composite material is the clear indication of better charge transfer across the interfaces and hence increased activity. Among the pure counterparts WO 3 has seen little charge drag was seen more conductive than ZnSe. The linear EIS plots of the materials dictate the capacitive behaviour of the electrochemical system. The kinetic parameters have been deduced by fitting data in Randle's Circuit (Inset Fig. 7). The values of simulated kinetic parameters have been presented in Table 2. It is clear from Electrochemical active surface area (ECSA). The active surface area of the nanostructures was determined by electrochemical double layer capacitance (EDLC, C dl ) (Fig. 8a,b). The active surface area was determined by executing cyclic voltammetry in non-faradic region. The current density (J) was plotted with scan rate (v) and from the slope of the plot double layer capacitance (C dl ) was deduced. The slope is considered to be equal to double layer capacitance which in turn is considered to be proportional to electrochemically active surface area. The Cyclic voltammogram's are depicted in Fig. 8a which advocates good capacitive behaviour of prepared nanostructures. From the linear plot Fig. 8b of current density (J) vs scan rate (v), the ECSA for WO 3 -ZnSe, WO 3 and ZnSe was calculated to be 45 mF cm −2 , 17 mF cm −2 and 9 mF cm −2 . It can be seen from the values that composite nanostructures has got increased surface area followed by WO 3 and ZnSe. The increased surface can lead to more active sites and enhanced photocatalytic activity. Moreover, the capacitive behaviour is also dependent on surface area and will increase upon surface increment.

Photoluminescence.
To spot the photoluminescence in the WO 3 -ZnSe nanocomposites, the sample was subjected to an RF spectrofluorometer. Figure 9a represents the PL spectrum and comprises two peaks. The WO 3 -ZnSe nanostructures display a low intensity PL signal due to slower recombination of electron-hole pairs in the range of 320 to 400 nm, which is responsible for enhancing the catalytic properties of the nanocomposites. The two emission peaks at 364 and 381 nm were attributed to electron-hole radiative recombination. Figure 9a shows the PL spectra of the WO 3 -ZnSe nanoparticles with an excitation wavelength of 240 nm. The  The starting and final concentrations are C 0 and C t , respectively. This proportion was plotted against the illumination time shown in Fig. 10b for the phenol solution treated with the WO 3 -ZnSe catalysts. From the concentration plots, it is obvious that the decrease in concentration is very rapid as soon as the catalysts are introduced into the solution in comparison to the blank solution. From the spectra Fig. 10a, it is visible that the phenol bands showed a gradual decrease without any shift. From the observations, it is determined that the concentration drop is due to the decolorization process. The declaration percentage was calculated using the following relation: After, 105 min of light illumination, the nanostructures of WO 3 -ZnSe decolored approximately 93.3% of the phenol. In two steps, the possible mechanism for photodecolarization can be explained. Highly reactive species (free radicals) are produced at first, and the next step is to have these free radicals oxidize the adsorbed phenol molecules (see Fig. 10). The catalyst is bordered by the OH molecules and reduces them to negatively charge when they are dispersed in the phenol solution. Due to electrostatic interactions, phenol is readily adsorbed across catalyst surfaces since it is weakly acidic. When exposed to a light source, photons with energy equal to or greater than the bandgap of the semiconductor are produced. Electron-hole pairs are formed as a result of the absorbed photons. Electrons are drawn to the conduction bands, while holes are drawn to the valence bands. The holes are consumed by absorbed hydroxyl groups, which are then converted into extremely active hydroxyl free radicals. Conduction electrons transform molecular oxygen to generate superoxides, which then react with water molecules to form peroxides.
Stability of the photocatalyst. The reuse test was utilized to measure the stability of the synthesized photocatalyst, and the outcomes are introduced in Fig. 11a,b for WO 3 -ZnSe nanocomposites. Even after the fourth cycle, the photocatalyst remained remarkably steady. The photocatalytic activity of both materials was reduced in the third cycle and thereafter remained relatively stable, according to the findings. The lower proportion of degradation may be related to successive degradation cycles.

Conclusion
Novel growth of WO 3 -ZnSe nanocomposites was carried out by a simple, low-cost hydrothermal process under subcritical conditions and is reported for the first time. The comprehensive morphological characterizations revealed the crystalline nature of synthesized nanostructures. The wide band gap nanomaterial was modified by making composites. The fabricated modified nanomaterial has a number of applications, exhibited higher crystallinity and outstanding photocatalytic activity due to the increased surface area after composite formation. The increased surface area was revealed by the electrochemical active Surface area analysis. Furthermore, the nanostructures display tremendous and marvelous properties toward charge storage, as demonstrated via cyclic voltammetry analysis. The synthesized material is suitable for blue LEDs employed by the International Commission on Illumination (CIE).